1. Field of the Invention
The present invention relates to a catalyst for exhaust gas purification.
2. Description of Related Art
As a catalyst for use in purifying exhaust gases from an internal combustion engine, a three-way catalyst has long been well known in the art. The three-way catalyst is able to simultaneously oxidize CO (carbon monoxide) and HC (hydrocarbons) and reduce NOx (nitrogen oxides). This three-way catalyst generally comprises .gamma.-alumina deposited with platinum (Pt), palladium (Pd) and the like and is known to exhibit a relatively high purifying efficiency with exhaust gases which have resulted from burning of an air-fuel mixture of about 14.7 in stoichiometric air-to-fuel ratio.
Of various noxious compositions contained in exhaust gases, NOx is generally considered to have an offensive impact on human bodies and environmental ecology. The general notion is that NOx emission to the atmosphere should be minimized. Numerous methods have been proposed to minimize NOx emission, and so long as automobile engines are concerned, the use of a catalytic converter on an engine exhaust system is widespread.
On the other hand, in the field of automobiles, a diluted combustion engine, that is, an engine operable with a leaned fuel, has been employed to meet regulations imposed on the fuel consumption. However, since the diluted combustion engine generally makes use of the air-fuel mixture of an air-to-fuel ratio higher than the stoichiometric ratio, the exhaust gases emitted from the diluted combustion engine are oxygen-enriched and, therefore, the three-way catalyst referred to above cannot satisfactorily reduce NOx even though it works well with CO and HC.
As a catalyst effective to decompose NOx, contained in the exhaust gases, into N.sub.2 and O.sub.2 directly or in the presence of a reducing agent (such as, for example, CO and/or HC) even under the oxygen-enriched atmosphere, a catalyst comprising a zeolite supporting a transition metal such as, for example, Cu by means of an ion exchanger is nowadays considered promising.
In order for the ion-exchange zeolite catalyst to exhibit a satisfactory NOx purification with respect to varying compositions of exhaust gases, studies have been extensively made on the composition of zeolite and active species carried by the zeolite carrier.
By way of example, Japanese Laid-open Patent Publication (examined) No. 3-89942 discloses a technique in which at least one rare earth element is carried by the zeolite together with Cu to allow the resultant zeolite catalyst to have moderate acid sites or acid centers. This prior art technique is intended to transform low-reactive paraffin hydrocarbons in the HC composition into high-reactive olefin hydrocarbons in the presence of the acid sites to thereby increase the activity with which NOx is decomposed.
However, the Cu-ion exchange zeolite catalyst generally considered useful to remove the NOx component effectively from the exhaust fails to exhibit satisfactory NOx purification under certain circumstances. More specifically, although the catalyst of the type referred to above generally exhibits a factor of 80% NOx purification under laboratory tests, the NOx purification factor tends to be lowered, when it is used in practice to remove the NOx component from the oxygen-enriched exhaust gases emitted from the diluted combustion engine, due to variation in characteristic of the exhaust gases.
Furthermore, in actual automotive vehicles, the exhaust gases occasionally come to have a high temperature close to 800.degree. to 900.degree. C. In the zeolite catalyst, such a high temperature has been confirmed to cause sintering of the transition metal employed as the active species, or to cause a structural change of, for example, removing Al contained in the zeolite from a crystal lattice, thereby lowering or eventually losing the activity of the zeolite catalyst.
On the other hand, under an automobile steering condition in which an automotive vehicle equipped with a diluted combustion engine, that is an internal combustion engine operable with an oxygen-enriched fuel, is repeatedly started and stopped, the temperature of exhaust gases emerging from such engine as measured at the inlet to the catalytic converter is low and, therefore, it has been desired to provide a catalyst having a sufficient resistance to the elevated temperature and also capable of exhibiting an increased exhaust purifying capacity at low temperatures.
In view of the foregoing, the inventors of the present invention have developed an NOx purifying catalyst wherein as active species to be carried by a metal-containing silicate such as, for example, zeolite, a noble metal including Pt and Ir is employed so that the resultant catalyst can have a high activity in the low temperature region. The inventors of the present invention have found that addition of Rh to the noble metal results in an increase of resistance of the NOx purifying catalyst to the elevated temperature.
In general, the NOx purifying catalyst is known to be of such a characteristic that, if a carrier matrix is made of zeolite, the purifying activity of the catalyst is high when the silica/alumina ratio is small, but lattice destruction takes place at a high temperature region accompanied by a reduction in activity and, therefore, optimum conditions can be found between the activity and the temperature of the exhaust gases at the inlet to the catalyst converter. On the other hand, if the silica/alumina ratio is relatively large, purifying activity tends to be is lowered while heat resistance increases, rendering it difficult for the catalyst to have both a satisfactory purifying activity and a satisfactory heat resistance.
Also, since at the high temperature region the activity and the degradation are affected by the temperature of the exhaust gases at the inlet to the catalytic converter and the atmosphere of the exhaust gases, it is often observed that the degradation of the zeolite used as the carrier matrix does not match with the degradation of the noble metal active species.
Accordingly, in order for the catalyst to satisfy two incompatible requirements to exhibit an excellent NOx purifying characteristic particularly at the low temperature region and to have an increased resistance to the elevated temperature, it appears that the metal-containing silicate such as zeolite should have a function of enhancing an initial activity of the catalyst and also a characteristic sufficient to increase the heat resistance.